Experimental infrared spectra of Cl−(ROH) (R = H, CH3, CH3CH2) complexes in the gas-phase

Abstract

Infrared multiple photon dissociation spectra for the chloride ion solvated by either water, methanol or ethanol have been recorded using an FTICR spectrometer coupled to a free-electron laser, and are presented here along with assignments to the observed bands. The assignments made to the Cl⁻/H₂O, Cl⁻(CH₃OH), and Cl⁻(CH₃CH₂OH) spectra are based on comparison with the neutral H₂O, CH₃OH, and CH₃CH₂OH spectra, respectively. This work confirms that a band observed around 1400 cm⁻¹ in the Cl⁻(H₂O) spectrum is not due to the Ar tag in Ar predissociation spectra. The carrier of this band is, most likely, the first overtone of the OHCl bend. Based on the position of the overtone in the IRMPD spectrum, 1375 cm⁻¹, the fundamental must occur very close to 700 cm⁻¹ and observation of this band should aid theoretical treatments of the spectrum of this complex. B3LYP/6-311++G(2df,2pd) calculations are shown to reproduce the IRMPD spectra of all three solvated chloride species. They also predict that attaching one or two Ar atoms to the Cl⁻(H₂O) complex results in a shift of no more than a few wavenumbers in the fundamental bands for the bare complex, in agreement with previous experiment. For both alcohol–Cl⁻ complexes, the S_N2 “backside attack” isomers are not observed and Cl⁻ is predicted theoretically, and confirmed experimentally, to be bound to the hydroxyl hydrogen. For Cl⁻(CH₃CH₂OH), the trans and gauche conformers are similar in energy, with the gauche conformer predicted to be thermodynamically favoured. The experimental infrared spectrum agrees well with that predicted for the gauche conformer but a mixture of gauche and anti conformers cannot be ruled out based on the experimental spectra nor on the computed thermochemistry.